1. Field of the Invention
The present invention relates to a zoom lens for use in cameras such as silver halide cameras, digital cameras and video cameras.
2. Discussion of the Background
With the increased spread of personal computers in recent years, the use of digital cameras as an information capturing device to capture image information for inputting into the personal computers has also widely spread.
Among various desires of the users for digital cameras, those for enhancing the image quality, for increasing the magnification, and for reducing the size and the power consumption of the cameras are strong. For satisfying such desires of the users, the photographing lens of digital cameras must have a zooming function, and at the same time, the photographing lens must be able to realize a high image quality corresponding to a resolution of a light receiving element, such as one having more than three million pixels, and that can realize the high magnification and the reduced size as well.
Also, it is preferable that a short distance focusing is possible for the sake of a user""s convenience. Specifically, it is desired that a relatively small area can be photographed in a short distance while keeping a high magnification.
A zoom lens suitable for being made compact is known that includes a first lens group having a positive focal length, a second lens group having a negative focal length, a third lens group having a positive focal length and a fourth lens group having a positive focal length. When zooming from a short focal point end toward a long focal point end using such a zoom lens, the second lens group monotonously moves from the object side toward the image side for changing the magnification, the third lens group monotonously moves from the image side toward the object side, and the focusing is accomplished by a movement of the fourth lens group. In this type of zoom lens, an auxiliary function of changing the magnification is performed by the third lens group, so that the moving distance of the second lens group for changing the magnification is small and the distance from the first lens group to the opening iris is shortened. Thereby, the lens diameter of the first lens group can be small. Examples of such zoom lenses are described, for example, in Japanese Patent Laid-open Publications No. 06-94997, No. 06-194572, No. 10-062687, and No. 11-119100.
In the zoom lenses described in JP No. 06-94997, No. 06-194572 and No. 11-119100, when zooming, the fourth lens group is required to be moved in order to keep the position of an object plane constant. That is, when zooming, at least three lens groups have to be moved. Accordingly, a plurality of driving actuators for zooming are needed, and further, a relatively large torque is needed for driving a cam that is used for moving the lens groups, which increases the power consumption.
Further, in actual products, because of variations in the dimensions of parts, if each lens group is disposed according the design, the object plane may deviate from the designed position when zooming. Such deviation of the object plane is avoided by adjusting the criterion positions of the lens groups when the products are assembled. Generally, the criterion positions of two lens groups are adjusted so that the object plane at the short focal point end coincides with the one at the long focal point end. If there exist more than three lens groups that are moved when zooming, when making the above adjustment, the lens group which is not used for the adjustment is also required to be moved, which undesirably increases the number of steps of assembling the products.
JP No. 06-194572 describes a zoom lens that can focus on an object substantially at a tip of the lens at the short focal point end. However, this is because the interval between the third lens group and the fourth lens group at the short focal point end when the focusing is made on an infinite distance is made sufficiently large such that the moving distance for focusing can be easily obtained. Therefore, if the size of the zoom lens is desired to be reduced, particularly if the total length of the zoom lens is desired to be reduced, the interval between the third lens group and the fourth lens group when the focusing is made on an infinite distance cannot be necessarily made sufficiently large so that the moving distance for focusing can be easily obtained. Accordingly, even if focusing is performed by movement of the fourth lens group, focusing cannot always be possible on an object at a tip end of the lens at the short focal point end.
In a zoom lens described in JP No. 10-062687, the focusing in the normal photographing area is performed by moving the fourth lens group and the focusing in a shorter distance is performed by moving the second lens group as well as moving the fourth lens group. That is, a plurality of lens groups are driven by a focusing signal from a distance measuring device, which undesirably complicates the driving mechanism.
The present invention has been made in view of the above-discussed and other problems, and addresses the above-discussed and other problems.
Preferred embodiments of the present invention provide a novel zoom lens and a camera using the zoom lens, that are compact and are capable of photographing a high quality image, and that are also superior in reducing the power consumption and that can be easily adjusted when assembled.
Preferred embodiments of the present invention further provide a novel zoom lens having a macro mode capable of focusing on a distance shorter than a normal photographing area and a camera using the zoom lens, that are suitable for being made compact and in which the macro mode mechanism is relatively simple.
According to a preferred embodiment of the present invention, a zoom lens includes a first lens group having a positive focal length; a second lens group having a negative focal length; an opening iris; a third lens group having a positive focal length; and a fourth lens group having a positive focal length. The first lens group, the second lens group, the opening iris, the third lens group and the fourth lens group are arranged in order from the object side. When zooming from a short focal point end toward a long focal point end, the second lens group monotonously moves from the object side toward the image side, and the third lens group first moves from the image side toward the object side and then, after arriving at a position closest to the object side in an area at a long focal point side beyond an intermediate focal point, moves toward the image side.
In the above zoom lens, the fourth lens group may be fixed when zooming. Further, the opening iris may be always fixed. Furthermore, the first lens group may be always fixed.
Further, in the above zoom lens, focusing can be achieved by a movement of the fourth lens group.
Furthermore, a focal length of the first lens group, f1, and a synthesized focal length of the first lens group and the second lens group at the long focal point end, f12T, may preferably satisfy a condition:
xe2x88x921.2 less than (f12T/f1) less than xe2x88x921.0
Still furthermore, the synthesized focal length of the first lens group and the second lens group at the long focal point end, f12T, a synthesized focal length of the first lens group and the second lens group at the short focal point end, f12W, a synthesized focal length of the first lens group through the third lens group at the long focal point end, f123T, and a synthesized focal length of the first lens group through the third lens group at the short focal point end, f123, W preferably satisfy a condition:
1.3 less than (f123T/f12T)/(f123W/f12W) less than 1.5
Also, each of the first through fourth lens groups preferably includes three or less lenses, and each of the second, third and fourth lens groups preferably includes at least one non-spherical surface.
According to another preferred embodiment of the present invention, the zoom lens includes a macro mode in which focusing is achieved on a shorter distance than in a normal photographing area. In the normal photographing area focusing is achieved by a movement of the fourth lens group. In the macro mode, the second and third lens groups are disposed in a positional relationship that is different from when zooming, and in the macro mode focusing is achieved by a movement of the fourth lens group.
In the above zoom lens, a position of the third lens group in the macro mode may be made so as to substantially coincide with a position of the third lens group at the long focal point end.
Further, a position of the second lens group in the macro mode may be made closer to an image plane than a position of the second lens group at the short focal point end.
Furthermore, when focusing is achieved on an infinite distance, the interval between the third and fourth lens groups may be made so as to be shortest at the short focal point end.
Further, a condition, f12T/f1 less than xe2x88x921, may be satisfied when f1 is a focal length of the first lens group and f12T is a synthesized focal length of the first and second lens groups at the long focal point end.